In communications apparatuses, bandpass filters act to pass only particular frequency bands with low loss, while blocking unnecessary high-frequency or low-frequency noises. As communications apparatuses used in portable wireless communications systems, etc. are miniaturized, laminated bandpass filters advantageous for miniaturization have become widely used (for instance, JP 2006-166136 A).
FIG. 23 shows the equivalent circuit of a bandpass filter described in JP 2006-166136 A, and FIG. 24 shows electrode patterns on layers in a laminated bandpass filter having the above equivalent circuit. This laminated bandpass filter comprises three one-side-short-circuited strip resonator electrodes 23a, 23b, 23c with alternating short-circuited sides formed in parallel on a sheet 4, three wavelength-shortening electrodes 22a, 22b, 22c, whose short-circuited sides are opposite to those of the strip resonator electrodes, formed on an upper sheet 3 at positions corresponding to the strip resonator electrodes 23a, 23b, 23c, and a capacitor electrode 28 formed on a lower sheet 5. The resonator electrodes 23a, 23c on the input and output sides are capacitance-coupled by the capacitor electrode 28.
As shown in FIG. 23, the center strip resonator electrode 23b is grounded on the upper side oppositely to the strip resonator electrodes 23a, 23c on both sides. With this difference, as shown in FIG. 24, the resonator electrodes 23a, 23c on both sides are connected to a ground electrode 29 on one side, while the center resonator electrode 23b is grounded on an opposite side. The laminated bandpass filter having the above structure, which is described in JP 2006-166136 A, is miniaturized with improved attenuation characteristics. However, because only signals in a necessary frequency band are permitted to pass in bandpass filters for wireless communications, there is increasingly higher demand for larger attenuation. Also, the miniaturization of wireless communications apparatuses has been increasing demand for further miniaturization of bandpass filters. However, the bandpass filter of JP 2006-166136 A does not have sufficient attenuation characteristics to meet the demand of miniaturization.
To adjust the filter characteristics of a bandpass filter while meeting the demand of miniaturization, JP 2002-16403 A discloses a dielectric filter having one resonator electrode having a different shape from those of the other resonator electrodes for controlling a resonance frequency without connection of a load capacitor. However, when the resonance frequency is adjusted only with the resonance electrode described in JP 2002-16403 A, change occurs not only in the resonance frequency but also in the degree of coupling between the resonators, resulting in the complicated adjustment of filter characteristics. When the shapes of the resonance electrodes are largely changed to adjust the resonance frequency, the area efficiency of the filter decreases, disadvantageous for miniaturization.
JP 2003-152403 A discloses a laminated bandpass filter comprising a first resonator comprising series-connected first transmission line and first grounded capacitor, a second resonator parallel-connected to the first resonator and comprising series-connected second transmission line and second grounded capacitor, a third resonator parallel-connected to the second resonator and comprising series-connected third transmission line and third grounded capacitor, and a coupling capacitor for coupling the first resonator and the third resonator, the main coupling of the bandpass filter being obtained by magnetic coupling between the first transmission line and the second transmission line and between the second transmission line and the third transmission line, whereby the coupling capacitor adjusts the frequency of an attenuation pole. JP 2003-152403 A specifically shows a circuit in which the grounded capacitor in the second resonator is disposed on the opposite side of the second grounded capacitor, and a circuit in which the grounded capacitor in the third resonator is disposed on the opposite side of the third grounded capacitor. This laminated bandpass filter achieves improvement in attenuation characteristics and miniaturization.
However, because both input and output terminals are DC short-circuited, the laminated bandpass filter of JP 2003-152403 A needs a DC-cutting capacitor. When used for portable communications apparatuses, etc., the DC-cutting capacitor should be mounted on a substrate, hindering miniaturization. When the DC-cutting capacitor is formed in the laminated bandpass filter, dielectric layers for forming this capacitor are needed, so that a laminated bandpass filter operated at 2.4 GHz, for instance; is 3.2 mm×2.5 mm×1.5 mm, larger than other circuit components mounted on a board, hindering miniaturization. In addition, when the reduction of area and height is sought only by the above structure, transmission lines become too close to the ground, resulting in reduced impedance of the transmission lines, and thus a poorer Q value with no load. Accordingly, steep filter characteristics shown in JP 2006-166136 A cannot be obtained.
JP 2003-152403 A describes that the position adjustment of the grounded capacitor connected to the resonator provides frequency compensation having an attenuation pole near the lower-frequency or higher-frequency side of a pass band. However, the generation of an attenuation pole on the low-frequency side fails to provide sufficient attenuation characteristics on the high-frequency side, and the generation of an attenuation pole on the high-frequency side fails to provide sufficient attenuation characteristics on the low-frequency side.